1. Field of the Invention
This invention relates to a process of producing dried wood chips made of the wood of broad-leaved and/or coniferous woody plants, particularly of wood from bushes and of waste wood or of wood obtained in wood-growing plantations.
2. Description of the Prior Art
Wood chips are used at increasing rates for heating purposes. Their heating value will strongly depend on the dryness of the wood chips. An adequate drying to a moisture content of 16 to 20% by weight will also be required to ensure that a sooting of the chimneys and an intolerable pollution of the environment by the flue gases from excessively moist wood chips will be avoided.
It is known that wood, particularly waste wood, can be predried in the air, but large drying areas are required, before the wood is chopped and the chips must be subsequently dried to the desired moisture content in drying plants heated by extraneous heat. But the chopping of predried wood is more difficult than the chopping of green wood. Predrying involves the performance of a plurality of operations between the supply of the wood and the chopping operation. Besides, storage times are required for drying and the final drying requires an expenditure of work, time and energy. Whereas a strong predrying will decrease the energy required for the final drying, the predried wood chips will be liable to be infested by pests or fungi when they are kept in intermediate storage under improper or unfavorable conditions and an undesirably high proportion of dust and fine particles may be formed in the chipped material and may even have to be removed for a production of wood chips which can be used in heating plants. Difficulties are involved in the dumping of the separated fines. In a search for alternative cultivation methods in agriculture, so-called wood-growing plantations have become significant, in which fast-growing woody plants, such as poplars, alders, willows and various species of Hibiscus, are grown in most cases as multiple-stem bushes. In some cases a trunk may be left in the ground and only the stems which have offshooted from the trunk may be cropped so that a plurality of harvests are possible in a multi-annual cycle without a need for a new planting. If the wood that has been cropped in such wood-growing plantations is to be used for the production of wood chips for fuel-firing furnaces, an economical utilization by which the costs are recovered or which is profitable is apparently impossible unless the wood chips produced can be dried without a need for extraneous energy. A large proportion of waste wood will also be contained in wood that has been broken by the wind or the snow and which must be processed as soon as possible in order to avoid an infestation by pests.